(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a method of preparing a positive active material for a rechargeable lithium battery with a simple process.
(b) Description of the Related Art
Rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated as positive and negative active materials. For an electrolyte, an organic solvent or polymer is used. Rechargeable lithium batteries produce electric energy from changes of chemical potentials of the active materials during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon material which intercalate lithium ions reversibly is extensively instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems which are present in the batteries with the metallic lithium can be prevented while achieving relatively higher energy density as well as much improved cycle life.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are reversibly deintercalated or intercalated are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1xe2x88x92xCoxO2(0 less than X less than 1) or LiMnO2. Manganese-based materials such as LiMn2O4 or LiMnO2 are relatively easy to prepare, are less expensive than the other materials, and have environmentally friendly characteristics. However, manganese-based materials have a low capacity. LiNiO2 is inexpensive and has a high specific capacity, but is more difficult to prepare. LiCoO2 is widely used in commercially available cells since it has good electrical conductivity and high battery voltage, but it is relatively expensive.
Generally, such composite metallic oxides are manufactured by the solid phase method. The solid phase method includes the steps of mixing solid raw material powders and sintering this mixture. For example, Japanese Patent publication No. Hei 8-153513 discloses a method in which Ni(OH)2 is mixed with Co(OH)2, or hydroxides including Ni or Co are mixed and the mixture is heat-treated, followed by the heat-treated mixture being ground and then sieved to produce LiNi1xe2x88x92xCoxO2 (0 less than x less than 1). In another method, LiOH, Ni oxide and Co oxide react and the reactant is initially sintered at 400 to 580xc2x0 C., and then the sintered reactant is sintered a second time at 600 to 780xc2x0 C. to produce a crystalline active material.
However, such conventional methods have shortcomings, namely they require complex multiple steps and various equipment, and a long duration time. The conventional methods are performed at relatively high temperatures and produce products with a relatively large particle size, and it is difficult to control physical properties such as morphology of the active material particle and surface characteristics (surface area, pore size). Furthermore, LiCoO2, which exhibits desirable electrical conductivity, good voltage and good electrochemical characteristics, is produced by heat-treating expensive oxides such as Co3O4 at 800 to 900xc2x0 C., resulting in an expensive product.
The physical properties of the active material are critical for the electrochemical characteristics and should be controlled to optimize characteristics of the battery. In other words, the characteristics of the battery depend on the physical properties of the composite metal oxide. However, with the solid-phase process it is difficult to control the physical properties of the active material.
It is an object of the present invention to provide a method of preparing a positive active material for a rechargeable lithium battery using a simple process.
It is another object to provide a method of preparing a positive active material for a rechargeable lithium battery inexpensively.
It is still another object to provide a method of preparing a positive active material for a rechargeable lithium battery which exhibits good cycle life characteristics.
These and other objects may be achieved by a method of preparing a positive active material for a rechargeable lithium battery. In this method, a lithium salt is reflux-reacted with a metal salt in a basic solution.
The present invention further includes a positive active material for a rechargeable lithium battery having a spherical or spherical-like form with a particle diameter of 10 nm to 10 xcexcm and a surface area of 0.1 to 5 m2/g. The positive active material is a compound represented by formulas 1 to 14.
where 0.5xe2x89xa6xxe2x89xa61.5, 0xe2x89xa6yxe2x89xa60.1, 0xe2x89xa6zxe2x89xa60.5; 0xe2x89xa6yxe2x80x2xe2x89xa60.5, 0xe2x89xa6zxe2x80x2xe2x89xa60.1, 0 less than xcex1xe2x89xa60.5;
Mxe2x80x2 is at least one element selected from the group consisting of Al, Co, Cr, Fe, Mg, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;
Mxe2x80x3 is at least one element selected from the group consisting of Al, Cr, Mn, Fe, Mg, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;
A is selected from the group consisting of O, F, S and P; and
B is selected from the group consisting of Ni or Co.